This paper presents a comparison of optimized mixed-mode and indirect-mode natural convection
solar dryers for maize. The mixed-mode and indirect-mode solar drying simulation
models were validated against results from a laboratory solar dryer with experiments carried
out under a solar simulator at the University of Newcastle upon Tyne, UK. The models are
now run under variable solar conditions in order to optimize the dryers and compare their
performance. The inputs to the simulation models were Zambian weather conditions and
materials. The solar drying simulations are combined with the cost of the dryer materials and
a search technique that finds the dryer dimensions at the minimum drying cost. Optimization
gave a shorter collector length for the mixed-mode solar dryer (1.8 m) than for the indirectmode
dryer (3.34 m) of the same grain capacity (90 kg). The drying cost, annual cost and
initial cost of the mixed-mode dryer are lower than those of the indirect-mode although the
quantity of dry grain obtained from the mixed-mode for the whole year is less than for the
indirect-mode; the drying costs are 12.76 and 16.05 US$/ton for mixed-mode and indirectmode
dryers, respectively. 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Indirect-mode; Mixed-mode; Natural convection; Optimization; Simulation model; Solar dryer